1. Field
The present invention relates generally to data communication, and more specifically to techniques for controlling the rate of data transmission for multi-channel communication systems.
2. Background
An orthogonal frequency division multiplex (OFDM) communication system effectively partitions the overall system bandwidth into multiple (NF) sub-bands, which may also be referred to as frequency subchannels or frequency bins. Each frequency subchannel is associated with a respective subcarrier (or tone) upon which data may be modulated. For an OFDM system, the data to be transmitted (i.e., the information bits) is first encoded with a particular coding scheme to generate coded bits, and the coded bits are further grouped into multi-bit symbols that are then mapped to modulation symbols. Each modulation symbol corresponds to a point in a signal constellation defined by a particular modulation scheme (e.g., M-PSK or M-QAM) used for data transmission. At each time interval that may be dependent on the bandwidth of each frequency subchannel, a modulation symbol may be transmitted on each of the NF frequency subchannels. OFDM may be used to combat inter-symbol interference (ISI) caused by frequency selective fading, which is characterized by different amounts of attenuation across the system bandwidth.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels may also be referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
For a MIMO system that employs OFDM (i.e., a MIMO-OFDM system), NF frequency subchannels are available on each of the NS spatial subchannels for data transmission. Each frequency subchannel of each spatial subchannel may be referred to as a transmission channel. NF·NS transmission channels are thus available for data transmission between the NT transmit antennas and NR receive antennas.
For a MIMO-OFDM system, the NF frequency subchannels of each spatial subchannel may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise-and-interference ratios (SNRs). Each transmitted modulation symbol is affected by the response of the transmission channel via which the symbol was transmitted. Depending on the multipath profile of the communication channel between the transmitter and receiver, the frequency response may vary widely throughout the system bandwidth for each spatial subchannel, and may further vary widely among the spatial subchannels.
For a multipath channel with a frequency response that is not flat, the information rate (i.e., the number of information bits per modulation symbol) that may be reliably transmitted on each transmission channel may be different from transmission channel to transmission channel. If the modulation symbols for a particular data packet are transmitted over multiple transmission channels, and if the response of these transmission channels varies widely, then these modulation symbols may be received with a wide range of SNRs. The SNR would then vary correspondingly across the entire received packet, which may then make it difficult to determine the proper rate for the data packet.
Since different receivers may experience different (and possibly widely varying) channel conditions, it would be impractical to transmit data at the same transmit power and/or data rate to all receivers. Fixing these transmission parameters would likely result in a waste of transmit power, the use of sub-optimal data rates for some receivers, and unreliable communication for some other receivers, all of which leads to an undesirable decrease in system capacity. Moreover, the channel conditions may vary over time. As a result, the supported data rates for the transmission channels would also vary over time. The different transmission capabilities of the communication channels for different receivers plus the multipath and time-variant nature of these communication channels make it challenging to efficiently transmit data in a MIMO-OFDM system.
There is therefore a need in the art for techniques to control the rate of data transmission in multi-channel communication systems such as MIMO-OFDM systems.